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Have a 90+ furnace where combustion testing outside is not practical or maybe even possible due to a roof termination? We searched for years for a cost effective and safe solution and found brass pipe plugs to be just that. Pipe plugs are tapered and therefore self sealing. They are also about 3/8" tall so they seal both walls of cellular core PVC. An inexpensive add-on per installation, a brass pipe plug installs quickly and makes testing year after year easy and safe. A much more professional look than silicone and also a much better seal. The acids in flue gas are so mild that they will have little to no affect on brass which has fair corrosion resistance to acids. Furnace condensate typically ranges between pH 3.0 and pH 5.0. By comparison, Orange Juice ranges from pH 3.30 to pH 4.15. After years of using them there is rarely even any oxidation of the plug.

NOTE these are 3/8" NPT (National Pipe Thread Taper) which means the hole size you drill needs to be 7/16"

1) Drill a 7/16" hole into the PVC pipe with a Vari-bit. You can check it as you go so you do not make it too big. This is slightly smaller than the standard tap size, but it will make a good tight fitting. Avoid getting too close to fittings.(Fittings means PVC fittings. Do not drill too close to a 90 degree PVC ell or similar fitting.)

2) Using an Allen wrench tap the hole in the PVC fitting with the plug, the plug will get tighter as it screwed in as it is tapered

Remove the plug and insert the combustion analyzer testing cone

Center the probe and lock down the set screw.

When finished testing screw the plug back into the test port. You can use Teflon tape or pipe dope approved for PVC if desired.